1. Technical Field
The present invention relates to a development device for use in an image forming apparatus such as a copier, a printer, a facsimile or a machine combining these, a processing unit and an image forming apparatus using the development device.
2. Description of the Related Arts
In an image forming apparatus such as a copier, a printer, a facsimile, or a machine combining these, an electrostatic latent image is formed on a latent image carrier, and a dry development device using powder developer for visualizing such an electrostatic latent image is widely adopted. As the developer, a two-component system developer having toners and carriers, a one-component system developer without having carriers, or the like is used.
This kind of development device generally includes a developer tank which is substantially sealed except for a portion facing a latent image carrier as a photoreceptor. The developer tank includes in the inside thereof, for example, a development roller which supplies toners to a photoreceptor, a supply roller which supplies toners to the development roller, and a control blade which controls the amount of toner supplied to the development roller. The development device also includes a toner container in which unused toners supplied from a toner supply bottle are retained. This toner container includes in the inside thereof an agitator which agitates and carries the toners. If the toners in the developer tank are used to form an image, the toners are supplied to the developer tank from the toner container by the agitator (for example, refer to Japanese Patent Application Publication No. 2008-275725).
The toners in the developer tank are stressed by passing through the contact portion of the development roller and the supply roller, the contact portion of the development roller and the control blade and the contact portion of the development roller and the photoreceptor in an image forming operation, so that an external additive added to improve the fluidity of the toners may be buried in the toners or may be separated, or the toners may be damaged or may be deformed, resulting in the decrease in the toner charge potential. If new toners in the toner container are supplied to the old toners in which the charge potential is decreased in the developer tank, the charge is interchanged between the old toners and the new toners by mixing the toners having different charge potential. As a result, the new toners are more highly charged higher than in general. In contrast, if the old toners are charged to a lower degree than in general, or charged in a reverse polarity, so-called surface staining, in which the toners adhere onto the background part (non-image part) of the photoreceptor, may occur, or variations in image concentration may occur.
In particular, if a lot of new toners are supplied to the developer tank from the toner container at one time, mutual charging between the toners rapidly occurs, so that the above-described surface staining and variations in image concentration tend to occur to a significant degree.